Marin Oaks

Oaks, Clouds - Marin County  2016

Oak, Rock - Marin County  2016

I could be wrong, but I believe that rock is serpentinite. As Wikipedia helpfully explains,

Serpentinite is formed from olivine via several reactions, some of which are complementary. Olivine is a solid solution between the magnesium-endmember forsterite and the iron-endmember fayalite. Serpentinite reactions  . . . exchange silica between forsterite and fayalite to form serpentine group minerals and magnetite. These are highly exothermic reactions.

Also, the State Rock of California.